The general objective of this research is to systematically investigate whether and how strength degradation in three major body joints--the shoulder, low back, and knee--affects the movement strategies and injury risk associated with the performance of manual materials handling. Our long-term goal is to develop quantitative tools and guidelines that integrate movement and strength information for the recognition, prediction, and prevention of occupational musculoskeletal injuries. This proposed research project seeks to accomplish the following specific aims: (1) To evaluate the strengths of human subjects, and through analysis of acquired strength profiles, to establish four subject groups one a symptomatic control group and three groups of individuals with selectively degraded shoulder, low back, or knee strength. (2) To measure three-dimensional movements during simulated manual materials handling tasks performed by the four groups of subjects under various task conditions (created by systematically varying task parameters such as load handled, angle of asymmetry, destination location). (3) To model the measured movements using a high-fidelity whole-body biomechanical linkage representation, and through determination of the active degrees of freedom in various measured movements, to examine the hypothesized whole-body joint motion activation pattern change. (4) To model the measured movements using an optimized-based differential inverse kinematics (ODIK) approach, and thus to visualize the hypothesized whole-body inter-joint motion apportionment strategy aberration. (5) To model the measured movements using an inverse-dynamics-driven direct dynamics (ID 4) approach, and thus to gain insight into the kinetic cause for hypothesized kinematics aberration and examine hypothesized changes in moment allowance (i.e., "strength reserve") and risk of injury. A successful completion of this project will lead to: (1) Motion-based evaluation of muscle strength degradation for proactive ergonomics intervention, return-to-work assessment, and rehabilitative ergonomics implementation; (2) Guidelines and computerized simulation models for designing consumer products or workplaces to better accommodate special population with degraded strength capability; and (3) A better understanding of how muscle strength influences the motion control strategies and consequently the injury risk during manual materials handling tasks in specific and human movements in general.